A hammer mill of this type is known from German patent 2713177. The shredding of the scrap charged, which is often bulky scrap such as automobile bodies or medium heavy mixed scrap, is effected both by the cooperation of the rotor hammers with anvils spaced from the striking circle of the hammers and by the impact of the material with the inside walls of the impact chute, against which it is flung. The rotor hammers, of which there may be any desired number in any desired distribution, may be mounted on freely rotatable axle rods arranged parallel to the rotor shaft and spaced apart around the periphery of the rotor. In this hammer mill the upper edge of the inlet opening forms part of a replaceable anvil with a gap between it and the striking circle of the hammers; a further anvil may be located on the lower edge of the side wall of the impact chute adjacent to the material outlet, and hence--viewed in the direction of rotation of the rotor--above the screening grid of the material outlet opposite the material inlet.
In operation, the hammers, with the anvil serving as counterpart tool, cut or tear pieces of material from the scrap metal supplied and fling these pieces against the walls of the impact chute, the bottom opening of which extends over the hammer rotor from the inlet to the outlet. This leads to deformation of, for example, impacting pieces of sheet metal and their separation from adherent impurities, and at the same time to compaction of the material, while more massive pieces of metal are deformed to a lesser extent but are likewise freed from adherent impurities before they again come within reach of the hammers before the outlet. Further shredding of the material can take place on an optional anvil of the outlet, whereafter the shredded material is ejected through the screening grid. Pieces larger than the width of the grid openings are dragged past the screening grid of the outlet and again subjected to the action of the shredding and counterpart tools.
To remove massive pieces of material that cannot be shredded to the size of the grid openings or smaller, and which advertise their presence in the hammer mill by loud noises, the operators must swing an ejection door in the impact chute inwards in the housing to a position in which it crosses the trajectory of the pieces, whereupon the door guides the pieces striking it to the exterior. Nevertheless it is not impossible for the massive problem material to become jammed in the space between the striking circle of the hammers and the screening grid of the outlet and/or the closed base of the housing. Less problems arise with smaller pieces, as the mass of the hammer can easily prevail on its own, i.e. the hammer can shred the piece that is jammed in the grid and/or compact it and pass it. When processing large, massive pieces of scrap, on the other hand, problems can arise which lead to quite serious breakdowns in operation.
When processing heavy material the rotor is usually driven relatively slowly. Particularly when the rotor is rotating only slowly a large piece that becomes jammed in the housing of the hammer mill in the region of the striking circle of the hammers may not be shredded and may not even be passed through because at the point of impact the rotor hammer may be in such a poor kinematic position that no deflecting rotary movement about its axle rod is possible. It also often happens that at least one of the hammers is stopped, at least for a short time, by the large and often heavy piece that is obstructing the striking circle. In these circumstances the forces of reaction that arise when the hammer is stopped by the large piece may, because of an unfavourable lever relationship, lead to fracture of the bearing axle or stub axles carrying the rotor hammers or of the rotor hammer itself, or even to bursting of the housing of the hammer mill.